VOLTAGE REGULATION Zener diode as a voltage regulator

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A Zener diode is a special type of diode that operates in the breakdown region to perform voltage regulation. The Zener diode provides a constant output voltage for an unregulated input supply voltage. The article details the zener voltage regulation and explores its advantages and disadvantages.

1. Using a Zener diode as a voltage regulator - this is how it works

Can a Zener diode be used as a voltage regulator?

A voltage regulator is a device that maintains a constant output voltage across the load irrespective of the variation in input voltage or load current. A Zener diode can regulate output load voltage because of its ability to generate the same voltage regardless of the input voltage or load resistance. Zener diode voltage regulator produces a stabilized output voltage with low ripples (AC component) for varying input unregulated voltage.

Zener diode as a voltage regulator

An unregulated input voltage V_in is the supply voltage to the circuit. A Zener diode is connected in reverse bias, parallel to the load with a resistor R_L in the circuit. V_O is the output-regulated voltage across the load resistor R_L and a resistor R is connected in series with the Zener diode to limit the excessive breakdown current I_z to a safer value. The Zener voltage V_z and the output-regulated voltage V_O across the load are the same (V_O = V_z).

The Zener diode begins regulation operation only when the input voltage (V_in ) is equal (or more than) Zener breakdown voltage (V_z). Otherwise, the diode remains “Off-state”.

V_in < V_z (Zener diode off)

V_in > V_z (Zener diode on)

Due to the parallel operation with load, Zener diodes are referred to as shunt voltage regulators. The Zener diode datasheet lists all the necessary values and characteristics for operation. An appropriate Zener diode can be chosen for regulatory requirements that must follow Zener breakdown operation. Also, the output load voltage must be chosen equal to the zener voltage.

The voltage-current relationship of the Zener diode is not linear throughout the reverse operation. It is because the Zener diode is a special device that operates well in the breakdown region.

Zener regulator operates in four configurations:

1. Input voltage V_in and load resistance R_L are fixed.
2. Input voltage V_in varies and load resistance R_L is fixed- Known as “line regulation
3. Input voltage V_in is fixed and load resistance R_L varies- Known as “load regulation
4. Both Input voltage V_in and load resistance R_L vary.

The calculations using Kirchhoff’s law and Thevenin’s theorem for fixed or changing values of V_in and R_L provides the value for V_O output load voltage. However, the zener current, and series resistor R are contributing factors for output load voltage calculation. The increase in the load current offers low load resistance and low load current increases the load resistance. The product of load current and resistance (as per Ohm’s law) provides a constant value of voltage against fixed or changing supply voltage or load resistance. Hence, the Zener regulator provides a constant output voltage VO for varying (or fixed) input voltage or load resistance.

2. When is it sensible to use a Zener diode as a voltage regulator?

It is necessary to use a Zener diode as a voltage regulator because the current across the load might increase heavily. The high value of source voltage may increase current across the load and cause overheating of the load (heating effect of current). Hence, to protect the load from overheating, the Zener diode acts as a protective circuit in the form of the “Zener Regulator”.

3. What are the benefits of using a Zener diode as a voltage regulator?

There are several benefits of using a Zener diode as a voltage regulator:

• Zener diodes operate in the breakdown region without damaging the circuit.

• Zener diodes protect the load from the heating effect of current.

• Zener diodes provide stabilized output voltage with low noise (AC components).

• Zener diodes are simple to design and connect to the circuit.

• Zener diodes are economically effective devices.

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4. What are the disadvantages of a Zener diode as a voltage regulator?

Zener diodes offer very few limitations:

• Zener diode can regulate voltage only when it is operating in reverse bias.

• If the input voltage is lower than the Zener voltage, the Zener diode remains off and behaves like an open circuit. The possible solution is to choose an appropriate Zener diode with a Zener voltage rating for the required voltage regulation.

• A Zener diode can operate in low-power applications only. Zener diodes have a low power rating, making them unsuitable for high-power industrial applications.

References

• http://vlabs.iitkgp.ernet.in/be/exp10/index.html

• https://youtu.be/kv0f-F3atgY

• Sharma, S.K., & Jerath, R. (2014). New Millennium Physics. S. Dinesh & Co.

• Boylestad, R., & Nashelesky, L. (2012). Electronic Devices and Circuits Theory (11th ed.). Pearson Education.

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